Split DB/tables to allow multi load and be flexible

This pull request is for https://github.com/fire015/flintstone/issues/4 and implements https://github.com/fire015/flintstone/issues/7 as well.

I've tried to keep the signatures as near as possible to the existing implementation, e.g. the DB itself only has a load method and not createTable(), removeTable() etc.
Same applied for the tables itself, but I've included some additional ones there.

The code may need some cleanup (phpDoc etc.) as I copied as much as possible from old code and was a bit tired.
I hope anyone else can do that!

Thanks & looking forward for your comment.